Process of electrically remelting high-melting metals

ABSTRACT

High-melting point solid metal is maintained immersed in an electrically conduction, electrolysable slag and is melted to form and maintain a pool of molten metal under said slag. An auxiliary electrode is maintained immersed in said slag. Unidirectional electric current is conducted through said auxiliary electrode and through said slag to induce fusion electrolysis in said slag so as to control ionic chemical reactions in a predetermined polarity direction taking place in said slag.

United States Patent m1 3,652,773 Holzgruber et al. [4 Mar. 28, 1972 ['54] PROCESS OF ELECTRICALLY 2,370,467 2/1945 REMELTING HIGH-MELTING METALS 2232.2? [72] Inventors: Wolfgang Holzgruber, Peter Junnergasse 3:024:35; 3 9 2 16, A8605, Kapfcnbers; Mlchner, 3,439,103 4/1969 Holzgruber et al. ...13/9 x Mullgasse 7, A8700, Leoben, both of Austria FOREIGN PATENTS OR APPLICATIONS [22] Filed: May 28, 1970 979,583 1/1965 Great Britain ..l3/9

[21] Appl' Primaryjxaminer-Bernard A. Gilheany v I Assistant Examiner-11. N. Envall, Jr. Attorney-Holman, Glascock, Downing & Seebold [62] Division of Ser. No. 676,094, Oct. 18, 1967, Pat. No.

3,571,475. 4 s71 ABSTRACT 52 us. Cl ..13/9,13/34 'i mm is mimimd electrically conduction, electrolysable slag and is melted to Search 13/9 9 ES 34 form and maintain a pool of molten metal under said slag. An 219/126 auxiliary electrode is maintained immersed in said slag.

Unidirectional electric current is conducted through said aux- [56] References Cited I iliaryelectrode and through said slag to induce fusion electrolysis in said slag so as to control ionic chemical reactions in a UNITED s1- ATES PATENTS predetermined polarity direction taking place in said slag.

2,368,998 2/1945 Nissim ..l3/9 7 Claims, 2 Drawing Figures PATENTEDmzs I972 sum 1 OF 2 INVENTORS I A'E'TORNEYS WOZFG'A/VG #ozzaeuase Para/e MACH/VEE' PATENTED ARZMHYZ 3,652 773 SHEEIEUFZ MENTOR;

AQORNEYS PROCESS OF ELECTRICALLY REMELTING HIGH- MELTING METALS This is a division of application Ser/No. 676,094 filed Oct. 18, 1967, now U.S. Pat. No. 3,571,475, and entitled Electroslog Refining Apparatus."

It is known that in electric slag refining processes the composition of the slag is of great significance for the performance of metallurgical reactions taking place between the steel and slag. Such reactions include desulfurication, removal of oxygen and others. Thus, the composition of the slag may have a decisive influence on the final composition of the refined steel. The extent to which such reactions take place is limited by physico-chemical and reaction-kinetical conditions. The slag may also be used to introduce various alloying elements into the steel. According to copending U.S. Pat. application Ser. No. 676,036, sulfur may be added to the slag so that a certain desired sulfur content of the final product can be obtained under certain conditions. It has also been proposed to increase the nitrogen content of the steel by the use of slags which release nitrogen.

A major difficulty in the performance of these processes resides in that the composition of the slag during the remelting operation changes continually as a result of chemicalreactions involving mainly the atmosphere which is in contact with the slag. ln accordance therewith, the tendency of the slag to take up or release given elements is continually changed in de pendence on the magnitude of the deviation from the equilibrium of distribution.

It is an object of the invention to enable a control of the slag composition and consequently of the interaction between the slag and the steel.

The invention is based on the fact that the slag is melted in the refining plant as a result of resistance heating occurring when an electric current flows through the slag and the slag becomes highly ionized. The inventive concept resides in that the chemical reactions between the slag and thesteel, and between the slag and the atmosphere, are influenced in the desired sense by electromotoric forces as electromotive forces are involved in these reactions.

The slag is highly ionized and the ions can be moved by a direct current flowing through the slag. The performance of metallurgical reactions between the steel and the slag can be influenced in this way. In all previous attempts .to utilize this influence in the electric slag refining process, the consumable electrode was of opposite polarity. to the ingot being formed in the mold. Hence, the fusible electrode was connected, e.g., as a cathode and the ingot as an anode, or vice versa. A true control of the performance of various processes is not possible with such circuit. This is clearly recognized if the principle of the process is recalled. The consumable electrode is immersed into the hot slag bath and is heated to the melting point. The drops of molten metal which are formed on the consumable electrode separate from the electrode and fall through the liquid slag and arecollected in a molten pool on the ingot which is formed in the cooled mold. When direct current is used, the drops being formed are at one polarity (e.g., cathodic) when they are still on the consumable electrode. As a drop subsequently falls down, it assumes the opposite polarity (e.g., anodic). Any chemical reactions which are performed on the anode in a predetermined direction will be performed in the opposite direction on the cathode. As a result, a drop of molten steel is first, e.g1, desulfurized and deoxidized on the consumable electrode by the action of the electric current, butwhen in the molten pool it picks up sulfur and oxygen from the slag. With opposite polarities, the steel would first pick up sulfur and oxygen from the consumable electrode and would release these two elements into the slag bath from the molten pool. In neither case can metallurgical processes be effectively controlled.

The present invention is based on the observation that the difficulties pointed out hereinbefore can be avoided if the consumable electrode and the ingot being formed are continuously held at the same polarity with respect to an auxiliary non-consumable electrode so that, e.g., the non-consumable electrode acts as an anode and the consumable electrode and the ingot serve as a cathoderWhen utilizing the present invention, it is possible, by the fusion electrolysis effect toproduce the undesired substances on the non-consumable electrode and to avoidthe transfer of desired metals or accompanying substances to the slag. i I

The present invention provides a process of making ingots of high-melting metals, particularly steel, by zone casting, in which at least one consumable electrode is fused down under the action of alternating current in an electrically conducting molten slag, and the invention resides in that fusion electrolysis is induced in the slag with the aid of at least one non-consumable auxiliary electrode, which is immersed in the slag to control the direction of metallurgical reactions taking place in the-slag.

FIG. 1 schematically shows a single-phase electric sla refining installation according to the present invention; and

FIG. 2 schematically shows a poly-phase electric slag refining installation according to the present invention.

As an example of an application of the present invention, the promotion of the desulfurization and deoxidation will be explained morein detail first.

For this purpose, at least one non-consumable electrode is required, which is immersed in the slag and is connected as an anode relative to the ingot and the consumable electrode. In this case, the consumable electrode and the bottom plate of the mold or the ingot being formed must be connected as a cathode. This arrangement results in the following reactions:

a. The sulfur S contained in the steel takes up two negative charges 2; from the cathode and is present in the slag as a negative ion S". The electromotive force exerted by the positive auxiliary electrode causes these S" ions to migrate to said auxiliary electrode so that the following reactions are obtained on the auxiliary electrode:

8'- S 2; S S S, S 20 S0, S0 and 8,, which is gaseous above 300 C., escape from the system at the anode.

b. The oxygen 0 in the steel takes up two negative charges from the cathode and is present in the slag as a negative ion 0''. The electromotive force of the positive auxiliary electrode causes these 0" ions to move to the auxiliary electrode so that the following reactions take place on the auxiliary electrode:

0" 0+ 2; 0+ 0 O, C 0 CO S 20 S0, 0,, CO and SO, escape from the system as a gas at the anode. The rates of desulfurization and deoxidation will increase with the velocity of ion migration. The lower limit of the electromotive force U, which moves the ions can be calculated from the parameters A H, T, A S and n which are typical of the reactions which have been described.

A H heat quantity which is consumed or released during a given reaction, in cal./mole A S change of entropy during a chemical reaction in cal./

C. mole.

The values of AH and AS are known for various metallurgical reactions and can be obtained from the literature.

T= absolute temperature in degree K.

n number ofelementary charges which are transferred per molecule or ion. The expression AH T X AS AG, defines the free enthalpy of the standard-reaction and corresponds to the work which is performed during a chemical process.

F Faradays constant, which indicates the quantity of electric current required for l/n mole lVal) F 96,500 (coulomb)= 26.6 ampere-hour The dc voltage U required for an electrolytic treatment of the slag during electric slag refining is generally calculated by the equation U U, JR

where U, theoretical electromotive forece (e.m.f. or E,,)

required to initiate the chemical process. This is calculated from 4) resistivity of the slag bath in ohm-centimeters 1 distance between electrodes in the slag in centimeters f electrode surface area in square centimeters.

An example will now be discussed. With the aid of the above expressions, the current and voltage will be calculated which are required for the discharge of sulfur and oxygen at certain reates.

EXAMPLE 'An ingot 300 mm. in diameter is remelted in an electric slag refining process at a melting rate of 200 kg./hr. To remove sulfur and oxygen, direct current is superimposed in the slag bath and eight non-consumable electrodes are used, each of which is 30 mm. in diameter. The slag has a resistivity of 0.5 ohmcentimeter. 0.010 percent oxygen and 0.020 percent sulfur are to be removed. The voltage and current required for this purpose will be calculated. The distance I between the electrodes is 50 mm.

Required rate of oxygen removal: 0.010 percent of 200 kg./hr. 20 g./hr. Required rate of sulfur removal: 0.020 percent of 200 kg./hr. 40 g./hr. Current required for removal of oxygen 20 g./hr./8 g. X 26.6 (ampere-hour) 66.5 amperes Current required for removal of sulfur 40 g./hr./l6 g. X 26.6 (ampere-hour) 66.5 amperes A constant total current of 123 amperes is thus required for removing sulfur and oxygen at the desired rates.

' A G for oxygen=- 35,860- 1.14 X Tat l,600 C., A G for oxygen 5 8,100 cal/mole A G for sulfur 62,520 5.27 X T at l,600 C., A G for sulfur 52,600 cal/mole I At low oxygen and sulfur concentrations, the expression R X T/n X F Xln K becomes so small that it can be neglected in the present calculation. Then, at l,600 C. U, for oxygen (58,l cal/mole X 4.2 ws/cal./2/mole X 96,500 coulombs) 1.5 volts U,, for sulfur (52,600 cal/mole X 4.2 ws/cal./2/mole X 96,500 coulombs) 1.1 volts In these calculations, the factor 4.2 is used for a conversion of calories into watt-seconds (ws).

In this calculation, a higher U was obtained for the oxygen so that this voltage must be used in the further calculations. From the above figures, the voltage U which is required in the process can be calculated as U U, R X J R= X l/f= 0.5 X /56 R 0.045 ohms U=l.3 +123 X 0.045 1.3 5.5 6.8 volts This shows that under the rcmelting conditions described above a dc voltage of at least6.8 volts and a current of 123 amperes are required to remove 0.020% S and 0.010% 0 from the steel.

The application of the invention is obviously not restricted to sulfur or'oxygen. Ionic reactions of all kinds can be controlled by electromotive forces as described above. Such reactions include, e.g., the removal of hydrogen from the steel if the hydrogen is present in the slag as OH ion.

On the other hand, when free-cutting steels are remelted and a transfer of sulfur from the steel into the slag or a chemical reaction of the sulfur with the slag is to be prevented, the non-consumable electrode must be connected as a cathode whereas the consumable electrode and the mold bottom plate constitute the anode. The resulting electromotive forces oppose the electromotive forces which wouldpromote an undesired removal of sulfur. The minimum voltage U, required for this purpose is also calculated from the relation E I A H- TA S/nF (volts) The non-consumable electrodes which are required in the process according to the invention and which are immersed in the slag may consist of metallic materials having melting points over 2,000 C., such as molybdenum or tungsten. The auxiliary electrodes consist suitably of graphite.

The non-consumable electrodes may be tubular and may concentrically surround the consumable electrode. Alternatively, uniformly spaced apartnon-consumable electrode rods may be arranged with their axes on the surface of an imaginary cylinder which concentrically surrounds the consumable electrode. It will be understood that a plurality of consumable electrodes can be melted at the same time. The process according to the invention can be carried out in an AC-operated electric slag refining plant, in which non-consumable auxiliary electrodes are arranged beside the consumable electrodes which are immersed in the slag. A dc voltage is applied between these non-consumable electrodes and the ingot and/or the consumable electrodes. An additional flow of direct current is thus produced in the slag and depending on the polarity, flows from or to the non-consumable electrode system. The dc voltage may be obtained from dc sources or in an AC-operated remelting system from rectifiers.

FIG. 1 shows a single-phase electric slag refining process according to the present invention. A single-phase transformer 1 has a secondary winding, which is connected at one terminal to the consumable electrode 2 to be fused down and at the other terminal to the bottom plate 11 of the mold and consequently to the ingot 3. This transformer supplies the energy which isrequired for the electric slag refining. The direction of current flow between the ingot and the consumable electrode is automatically reversed at the frequency of the supply system. Non-consumable auxiliary electrodes 5 are immersed in the slag 4 and used to superimpose a direct current on this alternating current. This direct current may flow to or from the non-consumable electrode system. The second pole of the direct current is connected to the consumable electrode, the ingot via the bottom plate, or both. The dc'voltage is derived from rectifiers 6. To impart to the consumable electrode the proper polarity relative to the non-consumable electrode, the terminal connected to the ingot is connected to the non-consumable electrode system by a rectifier so that the positive or negative half-wave is effective, depending on the forward direction of the rectifier. The terminal connected to the consumable electrode is similarly connected to the non-consumable electrode system. A switch 7 is provided for a reversal of the polarity of the auxiliary electrodes. A rheostat (variable resistor) 8 may be connected to control the value of the current flowing through the non-consumable electrode. The current carried by the non-consumable electrode results in a local temperature rise in the slag bath. This temperature rise may change the form of the molten metal pool on the ingot. The

switch 9 serves for supplying alternating current to the auxiliary electrode system when this is required.

The current value may be controlled by the resistor 8 and by the position of the non-consumable electrode system 5 in the slag 4 relative to the ingot and consumable electrode. At a given distance from the consumable electrode to the ingot, the distance of the non-consumable electrode to the consumable electrode and the ingot will determine the current flow through the non-consumable electrode according to Faradays Laws. The ratio between the partial currents flowing from the non-consumable electrode to the consumable electrode and to the ingot can be determined by selecting the distances from the non-consumable electrodes to the consumable electrodes and to the ingot.

FIG. 2 shows how the non-consumable electrodes can be connected in a polyphase electric slag refining process. The secondary phase windings of the transformer l which are connected to the consumable electrodes are connected by rectifiers 6 to an non-consumable electrode system 5. If the poly-phase remelting plant is'connected in star, another rectitier 10 may be connected between the neutral point and the non-consumable electrode. A changeover switch 7 serves for reversing the polarity of the non-consumable electrodes. The current through the non-consumable electrodes is controlled by the position of the non-consumable electrodes in the slag and by the rheostat 8. 1

Although the invention has been disclosed and illustrated in detail, it is to be understood that this does not delimit the invention. The spirit and scope of this invention is limited only by the language of the appended claims.

What is claimed is:

1. A process of remelting metals of high-melting point which comprises discharging alternating current through electrically conducting electrolysable slag positioned between a consumable electrode and a bottom element of an ingot mold; melting said consumable electrode to form a pool of molten metal under said slag; and concurrently discharging direct current through said slag between at least one non-consumable electrode representing one pole, and said consumable electrode and said pool of molten metal representing the other pole to enable fusion electrolysis of said slag so as to control the chemical reactions taking place in the slag.

2. A process as set forth in claim 1 wherein said consumable electrode consists of steel.

3. A process as set forth in claim 1 and further comprising the stop of selecting the direction of current flow through said slag so as to cause the undesired substances contained in the molten metal produced by melting said consumable electrode to be removed at said non-consumable electrode.

4. A process as set forth in claim 1 and further comprising the step of selecting the direction of current flow through said slag so as to prevent a transfer of substances which are desired in said pool into said slag.

5. A process as set forth in claim 1 wherein said non-consumable electrode consists of graphite.

6. A process as set forth in claim 1 wherein said non-consumable electrode is tubular and concentrically surrounds said consumable electrode.

7. A process as set forth in claim 1 wherein said non-consumable electrode consists of a plurality of electrode rods, which are evenly spaced apart and the axes of which are disposed on the surface of an imaginary cylinder which concentrically surrounds said consumable electrode. 

2. A process as set forth in claim 1 wherein said consumable electrode consists of steel.
 3. A process as set forth in claim 1 and further comprising the step of selecting the direction of current flow through said slag so as to cause the undesired substances contained in the molten metal produced by melting said consumable electrode to be removed at said non-consumable electrode.
 4. A process as set forth in claim 1 and further comprising the step of selecting the direction of current flow through said slag so as to prevent a transfer of substances which are desired in said pool into said slag.
 5. A process as set forth in claim 1 wherein said non-consumable electrode consists of graphite.
 6. A process as set forth in claim 1 wherein said non-consumable electrode is tubular and concentrically surrounds said consumable electrode.
 7. A process as set forth in claim 1 wherein said non-consumable electrode consists of a plurality of electrode rods, which are evenly spaced apart and the axes of which are disposed on the surface of an imaginary cylinder which concentrically surrounds said consumable electrode. 